Explosive growth of Internet services gradually boosts development of line transmission rates of an optical transport network from 40 Gbps to 100 Gbps, and then to 400 Gbps and even 1 Tbps. In a high-speed optical transmission system, a high-order modulation manner may be used to obtain higher spectral efficiency. For example, frequency-shift keying (FSK), differential phase shift keying (DPSK), quadrature amplitude modulation (QAM) or another high-order modulation manner may be used.
In an optical transmission system with a rate of or lower than 100 Gbps, modulation manners of on-off keying (OOK)/quadrature phase shift keying (QPSK) are used. A forward error correction coding (FEC) process and a modulation process in a transmitter are separately performed, so are decoding and demodulation performed in a receiver. In a next-generation higher-speed system, in order to more effectively utilize bandwidth and power, coding and modulation must be treated as an entirety, and therefore, a modulation and coding technology that is designed by combining high-order modulation and FEC coding becomes a critical technology in a high-speed optical system. Multi-level coding (MLC) is a high efficient modulation and coding scheme, which neither increases signal bandwidth nor reduces an effective data rate, thereby effectively improving data transmission performance.
In an MLC system in the prior art, a coding apparatus in a transmit end classifies bits of information into different levels, where FEC with a different bit rate is used for coding protection on each level, and then information obtained by mapping is sent to a receive end after each level is mapped to a high-order modulation constellation diagram. A decoding apparatus in the receive end separately performs FEC decoding on the different levels for bits that are obtained by demapping the received information. In a multi-level decoding (MSD) manner, information that is obtained by decoding low order bits is transferred to a demapping unit of next-level decoding, which reduces a BER of the next-level demapping, and then the next-level decoding is performed.
In the prior art, a soft-decision error correction coding (SD FEC) manner is used for each level in the MLC system. Implementation of this manner is very complex. Especially in application of a high-speed optical transmission system, an FEC overhead is subject to a greater restriction (approximately in a range of 7% to 30%). In this case, an overhead allocated to high-bit-rate FEC used to protect an MSB is very small. Highly complex SD FEC combined with a large throughout results in a very large quantity of resources for implementation of the entire system, which is not implementable.